PHP over VPN times out but static is OK?

This is an odd thing, so I hope I am able to describe it correctly. :)

I have an Apache 2.x setup serving basic intranet type apps. It works very well, and has so (in various configurations) for years. Where it gets weird, is when users try to access the apps over VPN from a remote site:

- 'static' HTML only pages are served correctly.
- 'PHP' scripts, depending on the contents, fail.

For instance, the following pseudo-code will be served:

<?php

$blah = "Hello World";
echo "The variable was $blah"; 

?>

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But this will time out over the VPN:

<?php

echo '<pre>';
print_r($_SERVER);
echo '</pre>';

?>

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As will this:

<?php

phpinfo();

?>

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So, it's not just simply that PHP scripts are not getting executed. All static HTML pages are served. Some PHP scripts are served, yet others are not.

As a reminder, the problem is ONLY over the VPN. On the LAN, there are no problems.

I don't think it is a DNS issue, since I get the same results by hitting both the IP address or the URL.

Any suggestions of where I can start looking? Or how I can test to see what is getting stomped on by the VPN?